An alternator of this type is known from U.S. application Ser. No. 198,967, filed Oct. 20, 1980, now U.S. Pat. No. 4,437,846 by the inventor hereof, assigned to the assignee of this application to which the German patent disclosure document DE-OS No. 29 42 737 corresponds. In the alternator described there, two ball bearings are employed to provide positive coupling of the rotor shaft with the fan wheel. The ball bearings are placed next to one another on the rotor shaft. The outer rings of these ball bearings are fitted into a bore of a hub formed, by casting, together with the fan wheel. The first outer ring, which is near the cup-shaped bearing plate of the alternator, is firmly fixed by a press-fit into the hub, while the other outer ring of the ball bearing, which is closer to the belt pulley, is able to slip in the bore of the hub when there is a difference in rotational speed between the rotor shaft and the fan wheel. It has been found, however, that for those materials usually employed in the casting of the fan wheel, the sliding friction causes the outlying ball bearing to loosen in the hub after a relatively short time period. This leads to unpredictable coupling torques.
The speed versus torque characteristic of the fan wheel and the drive is illustrated in FIG. 9, in which the abscissa represents speed of the shaft of the alternator, and the ordinate, torque. As the shaft starts to rotate, the fan wheel of the alternator will be carried along by the friction torque of the bearings. Upon increase of speed, the fan speed gradually also increases. If the speed n of the alternator shaft remains low, the fan wheel, shortly thereafter, will also reach essentially the same speed. Up to a maximum value, which, in general, is determined by spring loading due to construction of the bearings, the fan wheel will rotate synchronously with the shaft of the alternator. The limiting speed of this synchronous rotation is shown in FIG. 9 at n.sub.1, and is generally determined by the essentially constant torque M.sub.K which is necessary to overcome the friction within the bearings. If the speed of the alternator shaft increases above the value n.sub.1, the speed of the fan wheel should not increase essentially. FIG. 9 illustrates the torque M.sub.K necessary to overcome bearing friction, which is essentially independent of speed, and the torque M.sub.L, which is required to drive the fan wheel. At the above-referred limiting speed n.sub.1 and at torque M.sub.1, the two torques M.sub.K =M.sub.L =M.sub.1, that is, the torques are equal. Thus, at the speed of n.sub.1, synchronous operation between the alternator shaft and the fan wheel is obtained. At higher speeds, the operation is asynchronous.
The fan wheel should not rotate in synchronism with the alternator shaft throughout its speed range. Internal combustion (IC) engines have widely varying speed ranges. Operating the fan at a lower speed has the advantage that the lifetime thereof, as well as the lifetime of the bearings, is substantially increased, since wear and tear is reduced. By not driving the fan with the highest speeds of which the alternator can be driven, the drive power of the fan likewise is reduced, which increases the overall efficiency of the IC engine operation. Keeping the speed of the fan wheel at a level below maximum speed thus results in saving energy. Additionally, since customarily the alternators are driven by a V-belt, the belt itself will have a longer lifetime. The noise level is reduced. Due to the decreased acceleration required, loading on the V-belt and on the engine likewise is reduced. The speed of movement of the fan to move the air required to cool the generator is usually substantially less than the maximum speed at which the fan would rotate if it were directly coupled to the shaft.